1. Field of the Invention
The invention relates in general to a liquid crystal display and a method for driving the display, and more particularly to a liquid crystal display having improved motion image quality, and a method for driving the display.
2. Description of the Related Art
Common displays can be classified into two types depending on the method for displaying images: an impulse type and a hold type. A cathode ray tube (CRT) display is an example of a conventional impulse-type display. In a CRT display, electrons are accelerated in a vacuum tube and collide with phosphor powder coated on the wall of the vacuum tube, causing the phosphor powder to emit light for displaying images. As shown in FIG. 1A, the intensity of the light gradually decays during each frame period, so that the brightness of the image is maintained for only a few milliseconds. In another example, a liquid crystal display (LCD) is a hold-type display. As shown in FIG. 1B, within each frame period, an image is shown when pixel data are written to the pixels, and the brightness of the image is maintained for an entire frame period until the next pixel data are written to the pixels.
In general, when a liquid crystal display is displaying motion images, due to its hold-type display mode, some parts of the display area will display a portion of a new frame while other parts of the display area (where new image data have not been written to the pixels) will continue to show a portion of a previous frame. When the liquid crystal display is viewed by an observer, because the display area shows a portion of a new frame and a portion of a previous frame, and because human eyes track motion images, the observed motion images will have blurred edges and residual images, thereby reducing the image quality.
To solve the problems mentioned above, typically a black image is inserted in the image display process of an LCD display to achieve an effect similar to that of a CRT display, thus improving the motion image quality. As shown in FIGS. 2A and 2B, a frame period is divided into a first sub-frame period and a second sub-frame period. During the first sub-frame period, pixel data voltages are used to drive pixels to cause the pixels to display a normal image. During the second sub-frame period, a black image is inserted by using black image voltages to drive the pixels. The black image is shown until pixel data for the next frame period are written to the pixels to cause a new normal image to be displayed. The display mode for the LCD display as shown in FIG. 2B is more similar to the display mode for a CRT shown in FIG. 1A.
Below are examples of methods for inserting a black image. In one example, black images are generated by flashing a backlight source. Due to the need for long periods of repeatedly switching on and off the backlight module, this method has the disadvantages of larger electricity consumption, reduced lifespan of the backlight, and higher production costs. When the timing of backlight flashing is not synchronized with the display signals of the liquid crystal display, double images can occur so that an observer sees double images at the edges of objects when watching the motion images.
Another example is a cyclic resetting driving design disclosed in U.S. Pat. No. 6,473,077, which uses a double-frequency method to insert a black image. Referring to FIG. 3, using a liquid crystal display having a 640×480 resolution as an example, a gate driver (not shown) sequentially outputs 480 gate signals G1 to G480 during a first half of a frame period to drive corresponding rows of pixels to receive pixel data and display a normal image. During a second half of a frame period, the gate driver sequentially outputs 480 black image gate signals Gb1˜Gb480. This allows a normal image to be displayed during the first half of the frame period, and a black image to be inserted during the second half of the frame period.
Although the motion image quality can be improved by using the method described above, twice the number of gate signals and twice the amount of image data are used so that two images can be shown within a frame period. This requires doubling an operation frequency of the liquid crystal display, which increases the cost of the scan driver and the data driver. In the double-frequency driving design, because half of the frame period is allocated to the black image gate signals Gb1 to Gb480, only half of the frame period can be allocated to the gate signals G1 to G480, so that the period for writing pixel data is also reduced by half (from TA to TA/2). This may cause the pixels to have incorrect gray levels due to insufficient charging, and there may be increased electromagnetic interference (EMI) due to higher driving frequencies.
Referring to FIG. 4A, in another example of a cyclic resetting driving method, a display panel is divided into a matrix panel region A and a matrix panel region B that are coupled to data drivers 4 and 5, respectively. Referring to FIG. 4B, in a first half of a frame period, a gate driver 6 sequentially outputs gate signals G1 to G240 to drive the pixel rows in the matrix panel region A to receive pixel data outputted from the data driver 4 to display an image. In a second half frame period, the gate driver 6 sequentially outputs gate signals G241 to G480 to drive pixel rows in the matrix panel region B to receive pixel data output from the data driver 5 to display an image. Also during the second half frame period, the gate driver 6 sequentially outputs black image gate signals Gb1 to Gb240 to drive the pixel rows in the matrix panel region A to receive black image signals outputted from the data driver 4 to display a black image. Using this signal driving method, although the duration of each of the gate signals G1 to G480 remains the same as the original TA value, dividing the liquid crystal panel into two parts that are coupled to different data drivers increases complexity of the driving circuits and the manufacturing cost of the display.
A third example of a liquid crystal display is disclosed in Japanese Patent No. 9127917. Referring to FIG. 5, each pixel 500 is coupled to data lines Ld1 and Ld2 that are coupled to outputs of data drivers 510 and 520, respectively. Each pixel 500 is also coupled to scan lines Ls1 and Ls2 that are coupled to gate drivers 530 and 540, respectively. A normal gate signal Sg is transmitted through the scan line Ls1 to drive the pixel 500 so that the pixel 500 receives normal pixel data Dp from the data line Ld1 to display a pixel image. A black image gate signal Sd is then transmitted through the scan line Ls2 to drive the pixel 500 to receive a black signal Db from the data line Ls2 to display a black image. This method adds a scan line and a data line to each row and column of pixels, respectively, and will increase the production cost of the display and reduce the aperture ratio of the pixel.